Notes on Geology, Cosmology, and Solar System Origins
Geology and the Planetary Environment (08/26/25) - What is Geology?
The study of materials that comprise Earth, the history of Earth, and the processes at work on Earth.
Geologists and the Geologic Record
The geologic record preserves evidence of Earth’s long history.
Fathers of Geology
James Hutton (1726-1797) and Charles Lyell (1797-1875).
The Principle of Uniformitarianism
The today-observable processes have worked in much the same way throughout geologic past.
Methods (as a framework for analysis)
Energetically - open system: absorbs energy from the Sun.
Chemically - closed system: exchange between all atoms on the planet.
Aesthetic Method
Based on concepts of beauty, harmony, and order.
Faith/Spiritual Method
Based on concepts of eternal truths, supernatural, and rituals.
Political/Social Method
Based on compromise, consensus, power, and community.
A Good Hypothesis: criteria for scientific quality
Testability: surviving tests can elevate a hypothesis to a theory.
Verifiability: reproductivity.
Predictive Power: does it matter? Is it common knowledge?
Theory (definition in this context)
An explanation for an observation supported by a large body of evidence.
Theory of Plate Tectonics
Unified observations from many fields (continental drift, seafloor spreading, fossil distributions) into a single working theory.
Integrates seismology and volcanology with geology.
Limits in Science (as discussed in the transcript)
Curiosity: which observations are appropriate to study?
Originality: new ideas vs. established ones.
Technology: what tools enable study?
Parsimony: simplicity in explanation.
Challenges unique to science relative to other fields
Problems with scale, missing data, and problems with time.
Temporal context of the lecture (date stamps in the transcript)
9/8/25 notes indicate ongoing lecture sequence.
Cosmology & The Birth of the Earth (09/02/2025)
Are we spinning?- Foucault (Jean-Bernard-Leon) demonstrated Earth spins on its axis.
Cosmology defined- Cosmology = the study of the structure and evolution of the Universe.
Earth’s model history (overview of seismic and astronomical shift in models)- Ptolemy ( Alexandria, Egypt, 100-170 C.E. ) – Geocentric model.
Renaissance: Copernicus (1473-1543) – Heliocentric model.
Galileo (1564-1642) – Telescopic observations challenged geocentric model.
Isaac Newton (1642-1727) – contributed to magnitude/scale understanding.
Sense of scale: Earth circumference and key distances- Circumference of the Earth: C \approx 40{,}008 \text{km}
Distance to the Moon: d_{\text{Moon}} \approx 382{,}260 \text{km}
Moon distance in terms of Earth diameters: ~$30 \times \,D_{\text{Earth}}
Distance from Sun to Earth (1 AU): d_{\text{SE}} = 149{,}600{,}000 \text{km}
1 AU in miles: ~$93{,}000{,}000 \text{mi}
The approximate circularity of orbits: “the orbit is not an exact circle,” so numbers are approximate.
Distances to the stars and the scale of the cosmos- 382,260 km (Moon) is contrasted with interplanetary scales and beyond.
1838: Alpha Centauri distance estimate
Alpha Centauri is about 4.085 \times 10^{13} \text{km} away (40.85 trillion km).
In light-time terms, Alpha Centauri is about 4.3 \text{light-years} away.
Light year as a distance unit
1 \text{ly} \approx 9.46 \times 10^{12} \text{km}.
Thus Alpha Centauri \approx 4.3 \text{ly} away.
The edge of the visible Universe
Lies over 13 \text{billion light-years} away (light from there began its journey ~$9 \text{billion years} before Earth existed).
The Modern Image of the Universe- Stars are not randomly distributed; gravity pulls them into systems called galaxies.
The Sun is one of over 3 \times 10^{11} stars in the Milky Way (the Galaxy).
The Milky Way is one of over 10^{11} galaxies in the visible Universe.
The Universe & Our Galaxy (09/04/25) - Do galaxies move with respect to other galaxies?
Does the Universe become larger or smaller with time?
Has the Universe always existed?
Spectrum of Solar Radiation (Earth) — Overview of the Spectrum (Slide)
Visuals and axis description (from the slide)- Iirradiance vs wavelength: measured in W \, / \, m^2 \, / \, nm.
Regions: UV, Visible, Infrared (IR).
Solar spectrum with atmospheric absorption bands (primarily due to water vapor, H2O).
The Sun’s surface temperature: T_{\odot} = 5778 \text{K} (blackbody approximation).
Reflected/modified spectrum at sea level after atmospheric passage (H2O absorption bands).
Wavelength range shown on the slide: approximately from \sim 250 \,\text{nm} to \sim 2500 \,\text{nm}.
Key takeaway- The atmosphere absorbs certain wavelengths, shaping the spectrum that reaches Earth’s surface.
How Did the Universe Form? (Doppler Effect, Redshift, Hubble)
Doppler effect (sound and light)- Change in frequency (or wavelength) of a wave due to motion between source and observer.
In cosmology, this manifests as redshift and blueshift for light from moving objects.
Red Shift vs Blue Shift- Red shift: lower observed frequency (longer wavelength) when objects recede.
Blue shift: higher observed frequency ( shorter wavelength) when objects approach.
Edwin Hubble’s observations- Distant galaxies exhibit redshift regardless of direction relative to Earth.
This implies that the Universe is expanding.
The expansion is not at a constant rate; acceleration/deceleration mechanisms are part of the model.
The Big Bang theory (widely accepted)- Origin: a hot, dense singularity that expanded rapidly.
Formation of matter building blocks: quarks, protons, electrons, and eventually hydrogen and helium.
Age of the Universe- Approximately t \approx 13.7 \pm \ 0.14 \text{billion years} \left(13.7 \pm \ 1\%\right).
Lines of evidence supporting the Big Bang and expansion- Discovery of the expanding universe (Hubble expansion).
Measured current rate of expansion and back-calculation to estimate age.
Modeling the formation of elements (nucleosynthesis) in the early universe.
Star evolution over cosmic time.
What’s next? Our Galaxy- Galaxy formation began around t \approx 13.7 \text{BYA} from globular clusters.
Galaxy type: spiral.
Disk spins due to conservation of angular momentum.
The Milky Way is actively interacting (cannibalizing a neighboring galaxy).
Current star formation rate: ~$7 \text{stars per year}.
Origin of Our Solar System (Nebular Theory)
Not during the Big Bang; Solar System formed around 4.5 \text{billion years ago}.
What is a nebula?- Interstellar clouds of dust and gases; mostly hydrogen and helium; also other elements.
Gravitational forces cause clumping.
Nebular Hypothesis- The solar system condensed from a rotating nebula.
Stars and planets form when gravity pulls gas, dust, and ice together to form a swirling disc; the center becomes a star.
Rings around the star condense into planetesimals, which combine to form planets.
Solar System composition and distance from the Sun- Chemical composition varies with distance from the Sun during formation.
Meteorites as records of origin and age- Stony meteorites provide information about the origin and age of the solar system because they have not melted.
Chondrites are a key type used for dating.
Earth dating and radiometric methods- Earth dated using long-lived radioactive isotopes.
Connections, Implications, and Concepts to Remember
Uniformitarianism as a guiding principle: present processes operated in the past.
Interdisciplinary connections: geology, astronomy, physics, chemistry (seismology, volcanology, radiometric dating, nucleosynthesis, orbital dynamics).
Real-world relevance: understanding natural hazards, planetary formation, space exploration, and the history of science (geocentric vs heliocentric models, the Copernican revolution).
Philosophical and ethical considerations in science- Tension between observational evidence and beliefs (e.g., faith-based frameworks vs scientific explanations).
The role of models and theories as approximations that guide inquiry and technology development.
Key methodological concepts- Testability, verifiability, and predictive power as criteria for hypotheses and theories.
Parsimony and the balance between curiosity, data availability, and technological capability.
Important constants and approximate values to memorize (for exam familiarity)- Circumference of the Earth: C \approx 40{,}008 \text{ km}
Distance to Moon: d_{\text{Moon}} \approx 382{,}260 \text{ km}
Sun–Earth distance (1 AU): d_{\text{SE}} = 149{,}600{,}000 \text{ km}
1 AU in distance terms: 1 \text{ AU} = 1.496 \times 10^8 \text{ km}
Alpha Centauri distance: d_{\alpha\text{C}} \approx 4.085 \times 10^{13} \text{ km} (\approx 4.3 \text{ ly})
Light year: 1 \text{ ly} \approx 9.46 \times 10^{12} \text{ km}
Age of the Universe: t \approx 13.7 \text{ BYA} \; (\pm 1\% \approx \pm 0.14\text{ BYA})
Distance to edge of visible Universe: \gtrsim 13 \text{ billion light-years}
Solar spectrum temperature proxy: T_{\odot} = 5778 \text{ K}
Important concepts and formulas in this set- Doppler shift formula (for light, proportional to velocity): z = \frac{\lambda{\text{obs}} - \lambda{\text{emit}}}{\lambda_{\text{emit}}} and for small v, z \approx \frac{v}{c}.
Redshift as a signature of cosmic expansion (Hubble's observation).
Big Bang as the origin of the Universe’s expansion and the first building blocks of matter.
Nebular hypothesis for solar system formation: collapse of a rotating nebula into a protoplanetary disk, with planetesimals forming planets.
Radiometric dating using long-lived isotopes and meteoritic records (evidence for age and formation timing).
Summary takeaways
Geology provides a framework for understanding Earth’s history via processes that operate over deep time, guided by uniformitarian principles and a broad methodological toolbox.
Cosmology situates Earth within the larger Universe, tracing the shift from geocentric to heliocentric models, the expansion of the Universe, and the timing of cosmic and planetary formation.
The solar system formed from a rotating nebula, with gravity driving disc formation, planetesimal growth, and differentiation by distance from the Sun, leaving a legacy of meteorites that inform dating and origin studies.